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IC 


8949 



Bureau of Mines Information Circular/1983 




New Techniques for Reducing 
Stopping Leakage 

By Robert J. Timko and Edward D. Thimons 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 8949 

// 

New Techniques for Reducing 
Stopping Leakage 

By Robert J. Timko and Edward D. Thimons 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 



^w^^^ 



o. 






Library of Congress Cataloging in Publication Data: 



Timko, Robert J 

New techniques for reducing stopping leakage. 

(Bureau of Mines information circular ; 8949) 

Includes bibliographical references. 

Supt. of Docs, no.: I 28.27:89'19. 

1. Concrete stoppings (Mining)— Airtightness. 1. Thimons, Kdward 
D. II. Title. III. Series: Information circular (United States. Bureau 
of Mines) ; 8949. 



~~T1:^^&5tU4- [TN304] 622s [622'.42'028J 83-600226 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Background 3 

Construction techniques 4 

Brush-on application of mortar 4 

Area preparation 5 

Air leakage tests 8 

Procedures 8 

Initial test 9 

Floor grout tests 9 

Six-month test 10 

One-year test 10 

Maintenance 13 

Discussion of results 15 

ILLUSTRATIONS 

1 . Sealed concrete block stopping 2 

2. Stoppings separating parallel entries 3 

3. Apparatus for testing joint strength of block sample 5 

4. Pouring grout into block to help seal floor beneath stopping 6 

5. Average time required to construct stoppings 7 

6. Average quantity of mortar required to construct stoppings 7 

7. Test setup for creating pressure differential across stopping 8 

8. Air leakage evidenced by drop in SFg concentration over time 9 

9. Air leakage through newly constructed stoppings 10 

10. Air leakage through stoppings after 6 months 11 

11. Air leakage through stoppings after 1 yr 11 

12. Mortar-filled trough used to seal floor and stopping face 12 

13. Brushing flexible sealant onto stopping face 14 

14. Air leakage through stoppings after maintenance 15 

TABLE 

1 . Joint strength of block samples 4 





UNIT OF MEASURE 


ABBREVIATIONS 


USED 


IN 


THIS REPORT 


cfm 


cubic foot 


per 


minute 


lb 




pound 


ft 


foot 








min 




minute 


ft2 


square foot 








ppb 




part per billion 


ft3 


cubic foot 








psi 




pound per square inch 


in 


inch 








yr 




year 


in3 


cubic inch 















NEW TECHNIQUES FOR REDUCING STOPPING LEAKAGE 

By Robert J, Timko and Edward D. Thimons 



ABSTRACT 

Because of leakage through and around permanent stoppings in under- 
ground mines, more air must be forced into a mine than would otherwise 
be required for ventilation. As power costs increase, costs resulting 
from air leakage add increasingly to the operating costs of mining. 

The Bureau of Mines evaluated four different stopping construction 
techniques based on the ideas that (1) airtightness could be enhanced 
by brushing rather than troweling on mortar sealant and that (2) modi- 
fied mortars (mortars containing glass fibers and other additives for 
increased strength and adhesion) would improve sealing performance. 

Air leakage tests comparing conventional to modified stoppings were 
done when the stoppings were built, after 6 months, after 1 yr, and 
following simple maintenance. It was found that stoppings can be built 
and maintained better if — 

The area where a stopping is to be built is properly 
prepared. Stoppings are constructed using the techniques 
devised by the Bureau. 

Stoppings are periodically examined for leaks. 

Stoppings found to be leaking, especially at the perim- 
eters, are resealed. 



^Physical scientist. 
^Supervisory physical scientist. 
Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



Ventilation is required in underground 
mines to dilute and remove hazardous dust 
and gases , and to provide a sufficient 
quantity of fresh air for those working 
underground. Air is made to move by cre- 
ating a pressure differential between two 
points. This differential is usually ac- 
complished by a fan, operating as either 
a positive (blowing) or negative (ex- 
hausting) air displacement device. 

In room-and-pillar type mining, air is 
directed to the working sections by in- 
stalling walls called stoppings (fig. 1). 
Stoppings are usually constructed of con- 
crete block and sealed with mortar. They 
are placed in crosscuts between parallel 
entries and separate fresh incoming 
air from dust- or gas-laden exhaust air 
(fig. 2). 



In many mines , less than 50% of the to- 
tal ventilation air entering the mine ac- 
tually reaches the working faces. 3 Most 
air bypasses its planned course by leak- 
ing through or around imperfections in 
stoppings. When this happens, the fan 
must work harder to induce more air to 
compensate for the loss in order to de- 
liver the required quantity to the face 
area. Since the cost of power needed to 
operate the large fans that pull or push 
air through a mine is increasing, the 

^Ramani, R. V. , R. Stef anko, and G. W. 
Luxbacher. Advancement of Mine Ventila- 
tion Network Analysis From Art to Science 
(contract H0 133040 , PA State Univ.). 
Volume 4: Sensitivity of Leakage and 
Friction Factors. BuMines OFR 123(4)-78, 
1977, 115 pp. 




FIGURE 1. - Sealed concrete block stopping. 















<1- 




-< 


















































KEY 

1=1 Stoppings 

► Intake air 

d — < Return air 
Beit 



Tracl< 



FIGURE 2. = Stoppings separating parallel entries. 



Bureau contracted for research to evalu- 
ate ways to reduce this waste of energy 
through better stopping construction and 



sealing techniques. The results of this 
research are presented in this report. 



BACKGROUND 



The Bureau entered into a contract with 
Mine Safety Appliance Research, Inc. 
(MSAR) to examine the problem of air 
leakage through stoppings and means for 
alleviating it. This investigation was 
carried out in coal mines , but the re- 
sults are applicable to any kind of un- 
derground mine in which concrete blocks 
are used for stoppings. 

An initial survey was done to determine 
what sealants were being used and how 
they were applied. Troweling on mortar, 
especially modified mortar, was the usual 
method for sealing stoppings in coal 
mines. Some problems were found with 



troweling mortar sealants on the vertical 
stopping surfaces. It was discovered 
that vertical surfaces could be more 
thoroughly coated by brushing on rather 
than troweling on the mortar sealant. 
Most commercially available mortar seal- 
ants can easily be made compatible for 
brushing. 

Laboratory examinations of the flexural 
strengths of several brands of modified 
mortar showed that (1) the performance of 
different brands of modified mortar did 
not vary appreciably and (2) the brush- 
on technique was the key to increased 
joint strength. Based on these findings, 



several stoppings were constructed in an 
underground mine where they subsequently 
underwent a 1-yr evaluation. Each stop- 
ping was sealed by the brush-on tech- 
nique, but it was found that more than 
just mortar application was required to 
ensure good long-term performance. In 
particular, it was necessary to prepare 
the surface where the stopping contacted 
the floor, ribs, and roof. This included 



removing broken surface material, con- 
structing some type of floor seal, and 
sealing the ribs and roof. Maintenance 
was also found to be necessary because of 
flexing or crushing of stoppings by com- 
pressive loads. Resealing techniques 
were developed that enable mine personnel 
to reduce air leakage without the expend- 
iture of much time or expense. 



CONSTRUCTION TECHNIQUES 
BRUSH-ON APPLICATION OF MORTAR 



The brush-on technique evolved after 
investigators visited several mines and 
watched laborers using trowels to mortar- 
seal stoppings. This appeared to require 
considerable skill and experience. With- 
out experience, the trowel proved to be a 
difficult instrument to use because con- 
siderable manipulation is required to 
transfer mortar from the mixing apparatus 
to the surface being coated. At times it 
seemed that more mortar was dropped than 
reached the stopping face. 

By substituting brushes for trowels, 
laborers could more easily apply mortar. 
Tests with several types of brushes 
showed that they needed to be stiff 
enough to hold mortar, yet flexible 
enough to spread the mortar across a 



masonry surface. A plastic-bristle 
whitewash brush performed best and was 
used in all subsequent experiments. 

In preliminary laboratory tests to 
evaluate the brush-on method, block sam- 
ples consisting of two half concrete 
blocks and one whole concrete block were 
prepared by brush-coating both faces 
only to simulate a dry-stacked assembly, 
or by brush-coating the faces and joints 
to simulate wet-wall construction. The 
samples were then placed on a labora- 
tory instrument (fig. 3) to determine 
joint strength. The results are shown in 
table 1. 

For both dry-stacked and wet-wall stop- 
ping construction, troweling is consid- 
ered the standard mortar application 
technique. Dry-stacked stoppings are 



TABLE 1. - Joint strength of block samples, psi 



Application method and mortar 



Average force to break 



Aligned 
joints 



Misaligned 
joints 



DRY-STACKED SAMPLES 



Troweled on 1 face: B-Bond. 
Brushed on both faces: 

B-Bond 

Genstar mine sealant mix. . 
Thoro System products: 

With glass fibers 

Without glass fibers.... 



3,247 

3,267 
3,306 

3,407 
3,287 



3,213 

3,433 
3,667 

2,853 
2,160 



WET- WALL SAMPLES 



Troweled on joints: Sakrete...., 
Brushed on joints and both faces; 

B-Bond , 

Genstar mine sealant mix , 

Thoro System products: 

With glass fibers , 

Without glass fibers... , 



1,650 

4,167 
4,387 

4,800 
4,213 



1,640 

3,300 
3,653 

NT 
3,553 



NT Not tested. 



Load cell 




Upper support 
member 



Concrete block 
test specimen 



Lower support 
member 



FIGURE 3. - Apparatus for testing joint strength 
of block sample. 

usually mortared on both faces only, 
while both faces as well as the joints of 
wet-wall stoppings are mortared. The 
most commonly used mortars are B-Bond 
mortar (for dry-stacked stoppings) and 
Sakrete mortar (for wet-wall stoppings). 
Table 1 shows how resultant joint 
strengths compared when these (troweled- 
on) mortars and other (brushed-on) mor- 
tars were used. 

Permission was obtained to construct 
stoppings using the brush-on technique 
in Rochester and Pittsburgh Coal Co.'s 
Urling No. 1 coal mine in western Penn- 
sylvania. Conventional stoppings used in 
this mine typically were built by dry- 
stacking hollow concrete blocks and trow- 
eling mortar on one stopping face. There 
was little area preparation to ensure 
airtightness. The investigators' initial 
modification to conventional stoppings 
was the substitution of the brush-on 
technique for troweling. Stoppings con- 
structed in this manner are referred to 
in this report as quick-build stoppings. 
As expected, brushing on glass-fiber- 
enhanced mortar was much easier than 
troweling. Laborers were able to seal 
locations around the stopping perimeter 
that were impossible to coat with 
trowels. 



AREA PREPARATION 

A chronic problem with conventional and 
quick-build stoppings is that after a 
time the perimeters leak air. To reduce 
or alleviate this leakage, research was 
concentrated on preparing the surfaces 
where stoppings contact the floor, ribs, 
and roof. 

Two methods of floor sealing were eval- 
uated. In the first, a trough was dug 
into the floor. Dry modified mortar was 
poured into the trench and mixed with wa- 
ter to form a footer. The first concrete 
block course was then laid on the still- 
wet mortar. 

The second method entailed no floor 
preparation. The first concrete block 
course was laid on broken floor material, 
and water glass (sodium silicate) was 
poured into the holes in the hollow- 
core concrete blocks (fig. 4). The water 
glass then spread into the broken mate- 
rial beneath the stopping. As the pres- 
sure differential across the stopping 
increased, air was forced beneath it, 
solidifying the water glass and forming a 
barrier against airflow. In order to 
successfully form a seal, the liquid had 
to penetrate deep enough to create a high 
resistance across the entire stopping 
base. 

The technique used to seal the ribs was 
similar to that used to seal the floor. 
Before beginning stopping construction, a 
mattock, pry-bar, or similar tool was 
used to create a channel in both ribs. 
Stopping blocks were then "keyed" into 
the channels with mortar, forcing the 
mortar into any void between the stopping 
block and the rib. This formed a solid 
structure from rib to rib. When the 
stopping was completed, a mortar cove was 
created by again forcing mortar into the 
rib corners , then feathering it out onto 
the stopping and rib surfaces. 

To seal the roof, wood wedges were 
first forced into the void above the 
stopping. These wedges were parallel 
to the stopping face and approximately 




FIGURE 4. = Pouring grout into block to help seal floor beneath stopping. 



1 in. back from the face. Mortar was 
then brushed into the 1 in. void, creat- 
ing a rounded cove. Excess mortar was 
feathered out onto the roof and stopping 
face. 

As previously mentioned, leakage was 
primarily around stoppings. In an effort 
to control perimeter leakage, the return- 
side perimeters of several stoppings were 
coated. To do this, one laborer remained 
on the return side during construction. 
The laborer brush-coated the return pe- 
rimeter and then squeezed through an 
opening that had been left in one upper 
corner. The opening was then closed from 
the intake side. Thus, the return perim- 
eter was also sealed except for one upper 
corner. 



In addition to quick-build stoppings 
three other types of stoppings with dif- 
ferent levels of complexity were built. 
Universal stoppings were dry-stacked and 
had a footer, keyed ribs, sealed roof, 
and one face sealed with one coat of mod- 
ified mortar. High-pressure stoppings 
were made of hollow-core wet-laid con- 
crete blocks; had a footer, keyed ribs, 
a sealed roof, and two brush coats of 
modified mortar on the face. In addition 
the return perimeter was sealed. Hybrid 
stoppings were made in the same way as 
high-pressure stoppings, except that 
solid-core blocks were used instead 
of hollow-core blocks. Hybrid stoppings 
were thought to combine the best quali- 
ties of all the other modified stoppings. 



Stopping sizes averaged approximately for each stopping type and the average 
h-l/1 ft high by 18 ft wide. Figures 5 quantity of mortar, in 50-lb bags, re- 
and 6 show the average construction time quired to seal each type. 




Conven- Quick- Universal High 
tional build pressure 

FIGURE 5. - Average time required to construct stoppings. 



Hybrid 




Conven- Quick- Universal High Hybrid 
tional build pressure 

FIGURE 6. - Average quantity of mortar required to construct stoppings. 



AIR LEAKAGE TESTS 



PROCEDURES 

In the tests described below, there was 
a sufficient pressure differential across 
the stoppings to quantify the air leak- 
age. However, when the differential 
across the stoppings is less than 0.1 in 
w.g., it must be increased by a fan-and- 
parachute setup placed on the intake side 
of the stopping (fig. 7) before the leak- 
age can be measured. The parachute is 
fastened to roof bolts, then inflated 
with a fan. As the inflation pressure 
increases, a differential is created 
across the stoppings. For these tests 
the f an-and-parachute setup was not 
necessary. 

Stopping air leakage rates are mea- 
sured using a tracer gas, sulfur hexa- 
fluoride (SFg), which is a nontoxic, col- 
orless, odorless, and chemically and 
thermally stable gas. It is easily dis- 
pensed in air and has found widespread 
acceptance as a means for measuring mine 
ventilation.'* 



'^Drivas, P. J., P. G. Simmonds, and 
F. H. Shair. Experimental Characteriza- 
tion of Ventilation Systems in Buildings. 
Environ. Sci. Tech., v. 6, No. 7, 1972, 
pp. 609-614. 



The sampling technique for all stopping 
examinations, except the floor grout, was 
to enclose a volume of air on the return 
side of the stopping. For the conven- 
tional stoppings, this was done by erect- 
ing a brattice curtain an arbitrary dis- 
tance from each stopping. The modified 
stoppings were built in the same cross- 
cuts and on the intake sides of several 
other conventional stoppings , hence the 
conventional stoppings enclosed the vol- 
ume. Sulfur hexafluoride was released 
between the new and conventional stop- 
pings. The volume between the stoppings 
and the brattice curtains was then calcu- 
lated and recorded. 

After the SFg was released and per- 
mitted to diffuse in the volume, one end 
of a 0.25-in-ID tube was placed in the 
volume; the other end was connected to a 
sampling pump and remained in the return. 
At predetermined intervals, a sample was 
drawn by inserting a Luer device with an 
attached 90%-evacuated, 0.61-in^ test 
tube into a hypodermic needle that had 
been pierced through the sampling tube 
wall. Any leakage through the stoppings 
resulted in a drop in SFg concentration 
over time. 

Data reduction involved injecting 
a 0.0061-in^ sample of SFg into a gas 



/^fc^^^ 




Leak 


^ — 

WTube 


/ \ V-'--'^^^ ^--"^^ ^^ y^ / 




fsFe^ 




L, 11^^//'^^/^^ 


Sample 


J\:-^y(x 






Fan 


V.^1/ 






L».=.= 



Parachute 
stopping 



Test 
stopping 



Curtain 



FIGURE 7. - Test setup for creating pressure differential across stopping. 



chromatograph. A concentration was de- 
termined and graphed with respect to time 
(fig. 8). The air leakage rate, Q, was 
calculated as follows: 



Ti-i2 
where Q = air leakage rate, 



(1) 



C] = SFg concentration at time Tj , 
ppb, 



and V = stopping-to-curtain volume 
(ft^). 

To make results from all stoppings com- 
parable, actual air leakage rates were 
converted to cubic feet per minute per 
hundred square feet of stopping area per 
inch water gauge pressure differential. 
A chronological discussion of each leak- 
age rate study follows. 

INITIAL TEST 



C2 = SFg concentration at time T2 , 
ppb. 



1,000 



100- 



Q. 



< 

a: 



o 

o 
o 

CO 




20 30 
TIME,nnin 

FIGURE 8, - Air leakage evidenced by drop in 
SF concentration over time. 



Conventional and modified stoppings 
were examined for airtightness with- 
in 1 month of construction. As expected, 
the conventional stoppings were leak- 
ing more than the modified stoppings 
(fig. 9). By simply brushing on the 
modified mortar instead of applying 
it with a trowel, air leakage rates 
through the stoppings were reduced 
by more than 50%. For all the extra 
time and materials necessary to con- 
struct the hybrid stoppings, they 
were only slightly more airtight than 
the more easily constructed universal 
stoppings. 

The most impressive result of this test 
was the airtightness of the high-pressure 
stoppings. Leakage rates through high- 
pressure stoppings averaged 93% lower 
than leakage rates through conventional 
stoppings. Comparison of the high- 
pressure and hybrid stopping results 
showed that nothing is gained by using 
solid-core rather than hollow-core con- 
crete block. 

FLOOR GROUT TESTS 

The pourable floor grout tests were 
performed on four stoppings already built 
just inby the intake air shaft. A brat- 
tice curtain was erected on the intake 
side of the stoppings, and an initial 
SFg test was run. The face and the 
stopping perimeter (except at the floor) 
were recoated, and another SFg test 
was run. The floor grout was poured 
into a trough at the base of each stop- 
ping, and another air leakage test was 
performed. 



10 




Conven- 
tional 



Quick- 
build 



Universal 



High 
pressure 



Hybrid 



FIGURE 9. = Air leakage through newly constructed stoppings. 



These tests attempted to show the im- 
provement in airtightness after each re- 
sealing technique. In two of four stop- 
pings there was improvement, but in two 
others, leakage increased after the floor 
grout was applied. The only explanation 
is that when the trench was opened in 
front of each stopping, voids were cre- 
ated in two of the stoppings that the 
pourable grout was not able to close. 
More research needs to be done on this 
technique. 

SIX-MONTH TEST 

Because various problems prohibited 
completion of the airtightness tests for 
conventional stoppings, all effort was 
concentrated on remeasuring the modified 
stoppings and comparing the 6-month re- 
sults with the initial leakage rates. 

All four types of modified stoppings 
showed a decrease in air leakage af- 
ter 6 months (fig. 10). Air leakrates 
decreased — 

27% through the quick-build stoppings. 

34% through the hybrid stoppings. 



37% through the universal stoppings. 

40% through the high-pressure 
stoppings. 

Structurally, all stoppings appeared 
sound with only fine hairline cracks evi- 
dent at some perimeters. 

To explain why leakage rates around 
modified stoppings decreased with time, 
it was assumed that any uncoated holes 
remaining in the blocks or joints were 
very small and that dust entrained in air 
passing into the imperfections tended to 
plug these holes. This appeared reason- 
able because dust tracks were found sur- 
rounding several small holes in a stop- 
ping face. 

ONE- YEAR TEST 

During the second 6 months, air leakage 
through all modified stoppings increased 
dramatically. Since the conventional 
stoppings lacked a 6-month test, the 
leakage increase from them spanned 1 yr. 
If, however, the conventional stoppings 
performed similarly to the modified ones, 
the increase in leakage from them was 



11 




< £ 
o 



Conven- 
tional 



Quick- Universal High 
build pressure 



Hybrid 



FIGURE 10. - Air leakage through stoppings after 6 months. 



also more dramatic during the second 
6-month period. 

Comparing figure 11 to figure 10, the 

largest increase in leakage rates oc- 



400 



350 



300- 



V 250 

o 
o 

S. 200 



150 



100- 



50- 













1 1 1 1 1 1 1 




- 


1 






— 






V//A 



Conven- 
tional 



Quick- 
build 



Universal High 
pressure 



Hybrid 



FIGURE 11,- Air leakage through stoppings offer 1 yr. 



curred with the quick-build stoppings; 
air leakage rose slightly less than 400%. 
Conversely, hybrid stopping leakage in- 
creased the least, about 125%. 

The increase in stopping leakage was 
attributed primarily to the low humid- 
ity effect of winter. Low humidity tends 
to shrink and crack mortar, as well 
as the wooden wedges used to secure 
stoppings against the roof. Cracks were 
visible at most stopping perimeters, 
especially along the stopping-roof junc- 
tions. Leakage was audible around sev- 
eral stoppings. 

Because of good roof and floor con- 
ditions, stoppings did not appear to be 
under compression. In fact, some stop- 
pings had separated slightly from the 
roof and ribs. Due to the low humidity 
or possibly even freeze-thaw cycles, it 
appeared that the mortar and wood wedges 
at the stopping-roof junctions had shrunk 
enough to cause the stopping to move 
slightly away from the roof, creating 
a gap for air. In addition, ribs had 
spalled due to atmospheric conditions, 
causing more air leakage through them. 
Because this degeneration could not be 
tolerated, the following maintenance plan 
was devised. 



12 




^ 




FIGURE 12. - Moriar-filled trough used to seal floor and stopping face. 



13 



MAINTENANCE 



Simple techniques were devised to re- 
duce or eliminate air leakage in older 
stoppings. The basic premise was to keep 
the maintenance as simple as possible, 
thereby minimizing the time required to 
complete the work. Unless there is ac- 
tive roof, rib or floor movement and the 
stopping face has fractured, most leakage 
through older stoppings will be at the 
perimeters. 

Recoating the roof and ribs created few 
problems. By the time maintenance was 
necessary, a pressure differential ex- 
isted across the stoppings. The extent 
of audible perimeter leakage determined 
how far out from the stopping perimeters 
the sealant coating would be spread. 
Most stoppings repaired required an 8- to 
12-in-wide coating. The sealant was 
brushed on the stopping surface, then 
feathered onto the roof and ribs. 

Air leakage was also pronounced along 
the floor. This was most evident with 
the quick-build stoppings , which have 
no footer. Floor material adjacent to 
all stoppings was badly fractured, making 
it impossible to seal leaks by simply 
brushing-on sealant. 

A trench a few inches deep was cleared 
at the base of several quick-build 
and universal stoppings. Dry modified 
mortar was poured into the trough, 
followed by a liquid latex and water 
solution (fig. 12) and the mortar and 
solution were thoroughly mixed to a 
honey-like consistency. Not only did 
this fill and seal holes beneath the 
stoppings, but the material was also 
scooped from the trough and brush-coated 
onto the perimeters and any holes in the 
stopping faces. 



Since stopping maintenance should en- 
sure airtightness over a long period 
(e.g., 1 yr or more), it was thought 
important to apply a face sealant that 
would remain airtight. Flexible sealants 
appear to be better suited for long-term 
air barriers. There are several flexible 
sealants being marketed, but because of 
restrictions placed on their use under- 
ground, not many are suitable for mine 
stopping work. A semi-liquid acrylic 
latex loaded with glass fibers was chosen 
for face maintenance. This sealant was 
brushed on two stopping faces (fig. 13). 
Its short-term performance was compar- 
able to mortar based sealants. Similar 
sealants packaged in ready-to-use pails 
may be better designed for rapid mainte- 
nance since they require no time for mix- 
ing. In addition, since it is a latex- 
based sealant, it should remain flexible 
when cured, possibly eliminating the need 
for continuous maintenance. 

RESEALING TESTS 

Stopping leakage test results after 
performing maintenance are shown in fig- 
ure 14. Comparison with the original 
leakage rate tests (fig. 9) shows the 
quick-build and universal stoppings to be 
at least as airtight as when new. 

The time spent resealing stoppings var- 
ied with the stopping type. Since quick- 
build stoppings leaked more, more time 
had to be spent maintaining them. The 
total time necessary to reseal each 
quick-build stopping averaged two worker- 
hours. The other stopping types took 
slightly less time. 



14 




FIGURE 13. - Brushing flexible sealant onto stopping face. 



15 




Conven- 
tional 



Quick- Universal High 
build pressure 



Hybrid 



FIGURE 14. = Air leakage through stoppings after maintenance. 
DISCUSSION OF RESULTS 



Three techniques designed to reduce air 
leakage through and around concrete block 
stoppings were examined. Two techniques, 
brush-coating mortar and area prepara- 
tion, were designed specifically for new 
stoppings. The third, recoating stopping 
faces and perimeters, was used to help 
return older stoppings to the low leakage 
rates achieved when they were new. 

The new techniques were used to build 
stoppings in a coal mine. Leakage tests 
were performed on modified stoppings as 
well as those conventionally found in the 
mine. Initial, 6-month and 1-yr tests 
showed the modified stoppings to be much 
more airtight. 

After 1-yr, all modified stoppings ex- 
hibited an increase in air leakage, 
though they still remained more airtight 
than the conventional stoppings. Simple 
and inexpensive resealing techniques were 



performed on several stoppings. All had 
airtightness values as good or better 
than the amount measured when they were 
new. 

These approaches were examined in an 
effort to simplify stopping construction 
and reduce air leakage. Their only con- 
straint is that the new techniques be no 
more expensive than conventional methods 
of construction. Research has shown that 
airtightness and longevity are enhanced 
when stoppings are properly built using 
the Bureau's techniques. 

It is important for the mining industry 
to realize that air leakage through and 
around stoppings requires more air to be 
induced than is really necessary. As 
power costs increase, the additional air 
required means higher operating costs and 
thus more overhead. 



INT.-BU.OF MINES, PGH., PA. 27124 



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